1. Field of the Invention
This invention relates to an apparatus for detecting a light beam to be used in synchronization, and, in particular, to a synchronizing light beam detecting apparatus particularly suitable for use in a light scanning apparatus, such as a laser printer in which a light beam is scanned over an imaging surface.
2. Description of the Prior Art
A laser printer is well known in the art, and a light beam is repetitively scanned across the width of a photosensitive member which is moved in a direction perpendicular to the scanning direction of the light beam. The laser printer typically uses a polygonal mirror for deflecting a light beam having image information toward the photosensitive member. The polygonal mirror is provided with a plurality of mirror surfaces around its periphery, and as it is driven to rotate, the light beam deflected by one of the mirror surfaces is scanned across the photosensitive member. In this manner, the light beam carrying image information is repetitively deflected over a predetermined angle by each of the mirror surfaces one after another so that an electrostatic latent image is formed on the photosensitive member in accordance with the image information carried by the light beam.
In general, the light beam defected by the rotating polygonal mirror is then focused onto the photosensitive member in the form of a spot by an image forming lens, typically a f.theta. lens. As the polygonal mirror rotates to continuously change the positional relation with the incident light beam, the reflected light beam scans over a predetermined angle so that the light spot moves along a straight scanning line, whereby the light spot scans across the travelling photosensitive member. Such a scanning operation needs to be synchronized, and for this purpose, the light beam deflected by the polygonal mirror to be directed toward the photosensitive member is detected at a point beyond a predetermined scanning angle by a photodetector and used as a synchronizing light beam for synchronizing the initiation of each scanning operation.
The polygonal mirror has two major detrimental factors which could adversely affect the timing of scanning operation. The first factor relates to the so-called mirror surface inclination of the polygonal mirror, and this is the factor which arises from the fact that the mirror surfaces of the polygonal mirror are not truly in parallel with the rotating axis of the polygonal mirror but they are most likely to be inclined with respect thereto mainly due to manufacturing and assembling tolerances. The second factor relates to the scatter in the distance between each of the mirror surfaces of the polygonal mirror and its rotating axis, which also mainly stems from the manufacturing and assembling tolerances. Therefore, there has been a need to provide a new technique which can rectify such problems so as to improve the scanning operation in such a light beam scanning type imaging apparatus.